Use of mobile communications networks has tremendously increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications network. The base stations typically comprise radio systems for relaying radio signals, including software and hardware components. The radio signals are typically relayed into a cell of the mobile communications network. The operators of the mobile communications network wish to reduce the costs of the base stations. It is one option to implement the radio system as an antenna embedded radio system in order to reduce the costs of the base station. Implementing the radio system as an antenna embedded radio system may comprise implementing components of the radio system on a chip. Real estate needed to house the hardware components of the base station is reduced when implementing the radio station as an antenna embedded radio station. Power consumption during normal operation of the radio system is substantially reduced when implementing the radio system as the antenna embedded radio system comprising hardware components implemented on the chip.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of the several techniques comprises beam forming capabilities in order to direct a beam relayed by the radio system in different directions to improve service coverage within the cells of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several ones of the antenna elements of the active antenna system. Calibration of transmit paths and receive paths is required to provide the defined phase and amplitude relationship between the beams. The calibration allows the estimation of a phase and amplitude deviation accumulated along the transmit path of the radio system. Likewise the calibration comprises estimating a phase and amplitude deviation accumulated along the receive paths of the radio system. In a second step the phase and amplitude deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be applied to the individual ones of the transmit paths to yield the defined phase and amplitude relationship between the individual ones of the transmit paths of the radio system in order to allow for beam forming techniques.
The beam forming techniques rely on accurate transmit power levels of the radio system. If the transmit power levels of the radio system are not set correctly, the beam forming will be deteriorated. Therefore it is of interest for the radio system to ascertain the transmit power levels within a range of, for example, ±0.5 dB. Likewise a relative ratio of the transmit power levels needs to be accurate. Typically the relative ratio of the transmit power levels needs to be substantially more accurate than the transmit power levels of individual ones of the transmit paths. In the prior art the accurate (relative ratio of) transmit power levels was achieved by design of the radio system. Such an approach relies on attenuating properties and/or gain changing properties of the different ones of the transmit paths to be substantially known. Likewise the attenuating properties and/or gain changing properties of the individual power detectors, one per transmit path, need to be known. With a power amplifier in at least one of the transmit paths failing, the beam forming capabilities are no longer provided by the radio system. Furthermore the prior art does not provide any means of monitoring the radio system to indicate whether or not all transmit paths are relaying according to prescribed transmit power levels other than the obvious provision of one power detector per transmitter.
In the prior art it is common to use a calibration signal generator in order to provide a calibration signal. The calibration signal is used to calibrate the phase and amplitude changes applied to the transmit paths in order to obtain the defined phase and amplitude relation between the transmit paths. Typically the calibration signal in the prior art is hidden within a payload signal that is to be relayed along the transmit paths. Alternatively a dedicated calibration signal may be used during idle times of the radio system. A disadvantage of the calibration signal being applied during idle times of the radio system is twofold. Firstly the calibration signal is visible to other radio systems and all the users within the cell of the mobile communications network. Therefore signal to noise ratio (SNR) would be deteriorated for the other radio systems and/or the users present within the cell. Hiding the calibration signal overcomes the disadvantage of unwanted calibration signals being relayed invisibly to the other radio systems and/or the users within the cell of the mobile communications network. Unfortunately the hidden calibration signal is of low SNR and therefore the calibration according to the prior art is difficult.
A scheme for a phase and amplitude calibration of the radio system not requiring a dedicated calibration signal is disclosed in a co-pending application of the applicant Ser. No.12/416,639 . It is to be understood that the present invention does not require a dedicated calibration signal; neither for the power calibration of the transmit radio signals nor for the calibration of the phase and amplitude changes.
European Patent EP 1120858 B1 to NTT discloses an adaptive array transceiver apparatus. In the NTT patent a local generator is used for generating the calibration signal. The NTT patent does not provide a measurement of the RF transmit power levels of the adaptive array transceiver apparatus.